Ebola only spreads through physical touch, and it can't be spread by people who aren't showing symptoms of infection, Stephan Monroe, deputy director of the National Center for Emerging &amp; Zoonotic Infectious Diseases at the U.S. Centers for Disease Control and Prevention, explained in a Monday afternoon news conference.

Those factors -- and the fact that most flights between West Africa and the United States require one or more stops in other countries -- make it unlikely that a sick person could make it all the way to America and spread the Ebola virus, CDC officials said.

Concern was raised over the weekend when it was reported that an Ebola-infected man from Liberia flew to Lagos, the capital of Nigeria, only to die soon after his arrival. Health officials are trying to track down any contacts the man might have had during his journey.

Still, Monroe believes that "the likelihood of this outbreak spreading outside of West Africa is very low," although countries in the region will probably continue to see cases.

The current outbreak is the deadliest ever for Ebola, causing more than 670 deaths and 1,200 infections in Guinea, Liberia and Sierra Leone, according to the World Health Organization.

However, the death rate from Ebola during the current outbreak is about 60 percent compared to 90 percent from prior outbreaks -- indicating that early treatment efforts have been effective, Monroe said.

A pair of recent cases have highlighted the ease with which people can hop an airplane after becoming infected with Ebola or having come in contact with an Ebola patient.

No one is certain how the Liberian man, Patrick Sawyer, was able to board a plane and travel to Nigeria. His plane had a layover in the West Africa nation of Togo, and health officials are now scrambling to trace anyone who may have been exposed to Sawyer at any of the three international airports through which he traveled.

Sawyer, a 40-year-old employee of the Liberian finance ministry, recently had a sister die from Ebola. He was vomiting and had diarrhea aboard at least one of his flights, according to the Associated Press.

U.S. doctors are also keeping an eye on the wife and children of a Texas doctor who was working in Liberia to help Ebola patients. The family returned to America last week, days before the Dr. Kent Brantly -- who stayed behind -- came down with Ebola himself.

"Out of an abundance of caution, the family members are currently on a 21-day fever watch," Monroe said.

Public health workers are also discussing whether to step up monitoring at airports. "The idea of whether additional border-based controls can be more effective is actively under discussion by the international community," Monroe said.

But despite these cases, the odds are extremely low that an Ebola infection could reach the United States and spread into a full-fledged outbreak, said Thomas Geisbert, an expert on the deadly virus and a professor of microbiology and immunology at the Institute for Infectious Diseases at the University of Texas Medical Branch in Galveston.

Ebola is not transmitted through the air, which makes it much harder to spread than respiratory diseases like the measles, influenza or SARS, Geisbert explained.

Instead, Ebola is spread through body fluids like blood, vomit or saliva. A person has to come into contact with those fluids to risk infection.

"Because it's not airborne, it would take very close contact with someone who is at an advanced stage of illness to become infected," Geisbert said.

It is possible for someone to be infected with Ebola but initially show no symptoms, allowing them to board a plane with no one knowing they carry the deadly virus. However, the virus can spread only if a person experiences symptoms that spread bodily fluids like vomit and blood, Geisbert noted.

"You have to be sick to shed a large amount of the virus," he said.

And even if Ebola did reach an American airport, this country is much better prepared to deal with an infectious disease than those in West Africa, he added.

"We have a very good public health infrastructure in the United States," he said. "We have isolation wards in hospitals in most parts of the country. I think it would be much easier to contain here."

Brantly tested positive for Ebola and is showing symptoms, Monroe said. A family physician from Fort Worth, Texas, Brantly has been in Liberia since October 2013. He serves as the medical director of the Monrovia case management center for Samaritan's Purse, a North Carolina-based aid agency.

A second American also has been infected with Ebola. Nancy Writebol, a hygienist working for a group allied with Samaritan's Purse, had been focused on decontaminating people who came and went from the Monrovia care center, to prevent further spread of Ebola. She has suffered a fever but showed no other symptoms, Monroe said.

More information

For more information on Ebola virus, visit the U.S. Centers for Disease Control and Prevention (http://www.cdc.gov/vhf/ebola/ ).

SOURCES: Stephan Monroe, Ph.D., deputy director, National Center for Emerging &amp; Zoonotic Infectious Diseases, U.S. Centers for Disease Control and Prevention; Thomas Geisbert, Ph.D., professor, microbiology and immunology, Institute for Infectious Diseases, University of Texas Medical Branch in Galveston; July 28, 2014, news briefing, U.S. Centers for Disease Control and Prevention

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